The investigator's overall goal in this project is to map the apolipoprotein B elevating level (BEL) gene by doing a genomic search in pedigrees with familial combined hyperlipidemia, with the long-term goal of identifying the apolipoprotein B (apoB) elevating level gene and examining its effects in familial combined hyperlipidemia. Complex segregation analysis supports the existence of BEL, an unmapped codominant gene with a large impact on apoB levels. BEL genotype predicts 56% of adjusted apoB levels, cholesterol and triglyceride elevations, and familial combined hyperlipidemia (FCHL). All individuals with 2 copies of the apoB elevating allele develop FCHL and approximately half of those with one copy also develop FCHL. The investigator's mapping strategy is to increase power to map the BEL locus using a pending 10 cM screen markers by 1) using a quantitative trait, rather than FCHL status; 2) identifying a relatively homogeneous subset of families; and 3) adjusting apoB levels for covariates. The investigators propose using a subset of 15 FCHL families, identified from a group of 77 FCHL families as being most consistent with an apoB elevating locus. They have identified factors that lead to a significant portion of variation in apoB level. Adjustment of apoB levels for these factors will also increase the power to map the BEL locus. Standard lod score and newer Monte Carlo Markov Chain methods of linkage analysis will be used. Successful regional mapping will be followed by fine mapping and candidate gene studies. Major specific aims are: Aim 1. To increase the power of the linkage analyses by jointly considering other factors that influence apolipoprotein levels. Aim 2. To stratify families for linkage analysis on the basis of phenotypic or genotypic information. Aim 3. To identify the chromosomal region containing the apolipoprotein B elevating locus. Aim 4. To collect uncaptured data which will improve the power for fine mapping and cloning of the apolipoprotein B elevating level locus. Aim 5. To refine the position of the apolipoprotein B elevating level gene(s). Aim 6. Subsequent work to identify the locus if positive evidence of linkage to a strong candidate gene region is obtained.